If you've just run out of piping bags and have cupcakes to frost you can cut a small hole in the corner of a freezer bag, run your piping tip through the hole, and fill the bag with frosting. If you've never heard of a piping bag you can just cut the corner and use that as the tip for your bag.

Instructional video weblog HowCast shares the tip. If you have a piping tip it works best, but you can get by just using a hole cut in the tip of the bag. If you use this technique you'll want to not fill the bag more than half full of frosting to avoid making a mess.